THE dangers of out-of-date medicines could soon be a thing of the past, thanks to a special cap which stops drugs bottles being opened after their sell-by date.

Many drugs are ineffective, or can be dangerous, after their expiry date, but, until now, there has been no reliable way of stopping patients from taking them.

Now, scientists at Salford University have invented and developed a special screwtop containing tabs of plastic which degrade over time.

The "life" of each bottle is set by changing the proportion of various additives to the plastic.

The tabs, which are made sensitive to light or air, are bio-degradable and break down over time, preventing the bottles from being unscrewed. The time-scale is set according to which additive is put into the plastic.

Dr Alex Williams, who runs a Masters degree course in industrial design, came up with the idea, when he was trying to illustrate to students how product innovation and business opportunity can be linked.

The invention has now been patented and has attracted interest from some of the world's largest pharmaceutical companies.

"I wanted a case study to show students how an idea is developed to become a commercial enterprise. I think this has real commercial potential, but it is just one of many developments to come out of here," he said.

Negotiations for commercialising and marketing the product are at a crucial stage.

"It is just one example of the enterprising nature in the University of Salford. Students and staff are encouraged through a special division of the university, Academic Enterprise," says Dr Rick Watson, enterprise development manager, whose department helps technology transfer between academia, industry and commerce.